-snip-
Hmm, this is odd.  Here's the three lines in debug.log from around when it happened:
2015-12-21 06:00:05 LevelDB read failure: IO error: /storage/Backups/bitcoin/data/chainstate/3047481.ldb: Permission denied
2015-12-21 06:00:05 IO error: /storage/Backups/bitcoin/data/chainstate/3047481.ldb: Permission denied
2015-12-21 14:06:29 Error reading from database: Database I/O error
-snip-